Vectors A Guideline For Motion

Transcription

1 AP Physics-1 Vectors A Guideline For Motion Introduction: You deal with scalar quantities in many aspects of your everyday activities. For example, you know that 2 liters plus 2 liters is 4 liters. The concept of volume has no other dimensions associated with it. Scalar quantities are not affected by anything other than their magnitude or sizes. Scalar quantities are added according to the rules of ordinary arithmetic. You have probably encountered vector quantities in your everyday activities but may not have been aware that you were dealing with a vector quantity. Things that need more than one number to describe them are vector quantities. Vector quantities are entirely different from scalar quantities. The vector quantities (displacement, velocity, and acceleration), which you have encountered in this class so far, have had magnitude and direction. Force is another vector quantity we will study. The units for force are pounds in the English system and newtons in the international system. The sum of two vector quantities depends as much on their directions as their magnitudes. The sum of a 12 N force plus a 12 N force can equal from zero to 24 N depending on their directions. In physics we often deal with vector quantities as well as scalar quantities. An understanding of both is fundamental to an understanding of the basic principles of physics. Suppose you are not going to be a physicist. Why should you be concerned about the difference between scalars and vectors? L.C. Epstein in his book, Thinking Physics, answers the question this way. Because many people, who are not physicists, for example bureaucrats and business people, are asked to classify things, categorize things, or set up measuring schemes. These people frequently try to put things on a 1 10 or A F scalar scale without first stopping to think about what they are classifying. Sometimes doing this really messes up what they are trying to do. For example, the popular measure of intelligence is related as one number called IQ. That implies that intelligence is a scalar. But is intelligence really a scalar? Some people have good memories but can't reason. Some people learn quickly and forget quickly (crammers). Intelligence depends on many things like ability to learn, ability to remember, ability to reason, etc. So intelligence is a vector, not a scalar! That is a vital difference and the failure to recognize it has hurt thousands of people. So physicist or not, you had better get the idea of vector and scalar straight in your mind. Performance Objectives: Upon completion of the readings and activities in this unit and when asked to respond either orally or on a written test, you will: Clearly demonstrate an understanding of the difference between a vector and a scalar quantity. Give examples of each. Distinguish between distance and displacement speed and velocity. Show an ability to add any number of vectors by the graphical tip to tail method. Recognize the commutative law as applied to vectors. Be able to apply elementary geometry and trigonometry to vector algebra. Recognize the difference between a drawing of the actual situation and a vector diagram for the head to tail addition. Recognize the independence of vector quantities. Accurately interpret vectors jointly and separately. State the requirement for equilibrium. State the meaning of equilibrant. Distinguish between equilibrant and resultant. Resolve any vector into its perpendicular components graphically and trigonometrically. Use the method of rectangular resolution to add multiple vectors. Resolve graphically and trigonometrically a weight vector into components perpendicular and parallel to an inclined plane. Recognize that steeper inclines produce a lower normal force. Textbook Reference Chapter 3.1 & 3.2 "A number alone is, however, insufficient for describing some physical concepts. The recognition of this fact marked a distinct advance in scientific investigation." -Albert Einstein ( ) and Leopold Infield ( )

2 Exercises and Problems: A vector, which is directed line segment, can be used to represent a vector quantity. The length of the vector represents the magnitude and the direction shows the direction of the vector quantity. Vectors can be added geometrically by drawing the vectors to scale in the appropriate direction and connecting them tip-to-tail. The sum of two or more vectors is called the resultant. The resultant is drawn from the tail of the first vector to the tip of the last added vector. Draw vector diagrams to solve problems Use a protractor, a sharp pencil, and a metric ruler. 1. A plane flying due north, at m/s is blown due west at 50.0 m/s by a strong wind. Find the plane's resultant velocity. 112 m/s 27 W of N 2. A hiker leaves camp and walks 10.0 km due north. The hiker then walks 10.0 km due east. a.) What distance does he walk? b.) Determine his total displacement from the starting point. 20 km 14 km 45 N of E 3. An airplane flies due west at km/h. At the same time, the wind blows it from the north at 40.0 km/h. What is the plane's resultant velocity? S of W 4. Two soccer players kick the ball at exactly the same time. One player's foot exerts a force of 60.0 N south. The other's foot exerts a force of 80.0 N east. Find the magnitude and direction of the resultant force on the ball. 100 N 37 S of E 5. What is the vector sum of a 65 N force acting due east and 90.0 N force acting due west? 25 N West (Solve 1 to 5 using Pythagorean Theorem and trig. functions.) 6. A sales person leaves the office and drives 20.0 km due north along a straight highway. A turn is made onto a highway that leads in a direction 30.0 north of east. The driver continues on the highway for a distance of 62 km and then stops. What is the totally displacement of the sales person from the office? 74 km 43 E of N 7. Two forces of 60.0 N each act concurrently on point P. Determine the magnitude of the resultant force acting on point P when the angle between the forces is as follows: a.) 0 b.) 30.0 c.) 60.0 d.) 90.0 e.) 135 f.) N 115N 104 N 85 N 46 N 0N 8. In problem 7, what happens to the resultant of two forces as the angle between them increases? 9. An airplane flies at 150 km/h and heads 30.0 south of east. A 50.0 km/h wind blows in the direction 25 west of south. What is the resultant velocity of the plane? S of E 10. A 60.0 N force acting 30.0 east of north and a second 60.0 N force acting in the direction 60.0 east of north are concurrent forces. Determine the resultant force E of N 11. A 60.0 N force acts 45 west of south. An 80.0 N force acts 45 north of west. The two forces act on the same point. Find the magnitude and direction of their resultant N of W 12. A 30.0 N force acting due north and a 40 N force acting 35 east of north act concurrently on point P. Find the magnitude and direction of their resultant E of N Solve 6-12 again, this time use the Law of Cosine and the Law of Sines to solve them.)

3 EQUILIBRIUM when two or more forces act concurrently on an object and their vector sum is zero, the object is said to be in equilibrium. The word concurrent is a physics term which means not only at the same time but also that the lines of action of the forces acting on the object intersect. Engineers are frequently concerned with the stability of structures and the forces that supporting girders must offset. To have stability the forces acting on one single object must be arranged in such a way as to produce no net force. The vector sum of the forces must be zero. A resultant force of zero is known as the first condition for equilibrium. To find the equilibrant of two or more concurrent forces, first find the resultant of the forces. The equilibrant is a force equal in magnitude to the resultant, but opposite in direction. 13. Find the resultant and equilibrant in each of the following: a.) 60.0 N up; 20.0 N down; 5.0 N up N up 45.0 N down b.) 10.0 N north; 30.0 N east; 10.0 N south N east 30.0 N west c.) 50.0 N north; 10.0 N east; 20.0 N west; 40.0 N south. 14 N 45 N of W 14 N 45 S of E We all talk about the "force of gravity." This is a force exerted on all objects and each of us as well when we are in the earth's gravitational field. This force is called weight, W, of the body. Like all forces weight is a vector quantity and always directed toward the center of the earth. The weight vector is drawn perpendicular to the surface of the earth or perpendicular to the horizontal. As you work with, or read about forces in physics you will see another word used quite often. This word is TENSION. Tension is used to describe the force exerted by a rope, a rubber band, a fishing line, or a spring scale on two different bodies at the same time. For example, a string used to hang a picture. The string exerts a force on the nail in the wall and on the picture. Another way of looking at tension is to think of it as the force transferred from one body to another by the rope, string, etc. A rope or string, which is usually flexible, is rigid and said to be "under tension" when transferring a force. 14. A picture is supported on a wall by a cord connected between two eyelets, one on either side of the picture, and hanging over a peg. At the peg, the cord on either side makes an angle of 30 with the horizontal. What is the tension in each cord if the picture weighs 20 N? T 1 = T 2 = 20 N 15. A 50.0 N picture is hung from a nail by two cords that make 45 angles with the horizontal. Find the tension in the cords. 35 N Use the following figures for chain T 2 T A 1000 N load hangs from the end of a boom as shown in the figure above. Find the pull (tension) of the chain and the push (compression) of the boom on the point from which the load is suspended if the angle between the chain and the boom is 45. boom 1000 N chain 1414 N 17. A weight of N is suspended as shown in the figure above. Find the tension in the supporting cables. T 1= 1050 N T 2= 1313 N

4 PLEASE NOTE!!!!!! VECTORS WHEN ADDED PRODUCE A RESULTANT VECTOR, BUT NEVER ALTER EACH OTHER'S VALUES!!! EACH VECTOR REMAINS COMPLETELY INDEPENDENT OF ALL OTHER VECTORS!!! THE RESULTANT MERELY INDICATES THE COMBINED EFFECT OF THE VECTORS!!! 18. A motorboat heads due east at 16 m/s across a river that flows due south at 9.0 m/s. a.) What is the resultant velocity of the boat? b.) If the river is 136 m wide, how long does it take the motorboat to reach the other side? c.) How far downstream is the boat when it reached the other side of the river? 19. A motorboat travels at 40.0 m/s. It heads straight across a river 320 m wide. a.) If the water flows at the rate of 8.0 m/s. What is the boat's velocity with respect to the shore? b.) How long does it take the boat to reach the opposite side? 41 m/s 8.0 sec 20. A boat heads directly across a river 40.0 m wide at 8.0 m/s. The current is flowing at 3.8 m/s. a.) What is the resultant velocity of the boat? b.) How long does it take the boat to cross the river? c.) How far downstream is the boat when it reaches the other side? 8.9 m/s 5.0 sec 19 m 21. An airplane is to maintain a velocity of 475 km/hr in a northeasterly direction. If the wind velocity is 50.0 km/hr southeast, what should be the magnitude and direction of the velocity of the airplane in order to offset the effect of the wind? 478 km/hr 39 E of N 22. An airplane s destination is miles east of its starting point. The wind is from the northwest at 30.0 miles/hr. The pilot wishes to arrive at his destination in forty minutes. What should be his heading and airspeed? 280 mi/hr 4 N of E 23. General Lee Friendly is to fly an airplane into enemy territory, 115 kilometers directly north of his present position. His plane travels at 85 km/hr when at full throttle, and is to cross over the enemy line in precisely 1.8 hours. The wind is blowing from the west at 55 km/hr. At what angle relative to the north direction should he aim his plane for this to work out? Is his aircraft capable of getting him to his destination on time? 40.7 YES! Perpendicular components of vectors A component of a vector is its effective value in any given direction. A vector may be considered as the resultant of two or more component vectors. It is customary and most useful to resolve a vector into its perpendicular components (This is sometimes called rectangular resolution). 24. A heavy box is pulled across a wooden floor with a rope. The rope forms an angle of 60.0 with the floor. A tension of 80.0 N is maintained on the rope. What force actually is pulling the box across the floor? 40.0 N 25. The rope Problem 24 is lowered until it forms an angle of 30.0 with the floor. A force of 80.0 N is maintained on the rope. What force pulls the box across the floor? 69 N 26. An airplane flies 30.0 north of west at km/h. At what speed is the plane moving a.) north? b.) west? 250 km/h 433 km/h 27. To cross a stream, which flows west to east, a boat steers at an angle of 60.0 north of west. The boat travels at 8.0 km/h in still water. What is the speed of the current and the boat's speed relative to the shore if a person on shore sees the boat come straight across the stream? Current is 4km/h Boat is 6.9 km/h 28. A river flows due south. A riverboat pilot heads the boat 27 north of west and is able to go straight across the river at 6.0 m/s. a.) What is the speed of the current? b.) What is the speed of the boat? 3.1 m/s 6.7 m/s

5 ANOTHER LOOK AT THE ADDITION OF VECTORS Another method for adding vectors is to resolve the vectors to be added into perpendicular components. Add all the horizontal component vectors, then add all the vertical component vectors. This produces two vectors acting at a right angle. Pythagorean Theorem can be used to find the resultant. This method is especially useful when you are adding many vectors. Of course, if you prefer, you can still add several vectors graphically or using the law of cosines. 29. Add the following displacements: 8.0 m east, 5.0 m 30.0 north of east, 7.0 m 37 west of north, and 3.0 m south. 9.6 m 32 N of E 30. Find the resultant force of the three vectors acting concurrently on point P: N 30.0 north of east, N 45 north of west, and 155 N 55 south of west. 225 N 55 north of west 31. Find the resultant of the following displacements: 19 m east, 15 m 60.0 north of east, 16 m 45 north of west, 11 m 30.0 south of west, and 12 m south. 8.9 m 50.0 N of E 32. Find the resultant of the following set of forces: N east, N 60.0 north of east, N 45 north of west, and N south. 308 N 25 north of east GRAVITATIONAL FORCE AND INCLINED PLANES The gravitational attraction of the each acting on an object, its weight, W, is directed toward the center of the earth. This means that its weight, W, must act perpendicular to the surface of the earth, or as we say perpendicular to the horizontal. When an object is on an inclined plane, the plane prevents the weight, W, from acting perpendicular to the horizontal. Instead, the plane causes the weight, W, to be resolved into two components. One component, F perpendicular, acts perpendicular to and into the inclined plane. The second component, F parallel, acts parallel to and down the inclined plane. As the incline becomes steeper, the component of the weight acting down on the incline becomes greater. The component of the weight acting perpendicular to the inclined plane becomes less. As a result, you find smooth inclines treacherous while walking particularly if they are steep. The normal force the force that the inclined plane places on whatever object may be sitting on it is equal in magnitude but opposite in direction to the perpendicular component of the object s weight. If the incline is steep enough, you may find yourself forced to run down its length when you had no intention of running. The force down the incline, F parallel, accelerates you against your will. 33. A N trunk is placed on an incline plane that forms a 30.0 angle with the horizontal. a.) Calculate the values of F N and F P. b.) Calculate the values for the components parallel and perpendicular to the inclined plane when the angle is increased to c.) When the angle of an inclined increases, how do the force components acting on the trunk change? 433 N 250 N 34. An automobile weighing 12,000.0 N is parked on a 37 slope. a.) What force tends to cause the auto to roll down the hill? b.) What is the normal force between the auto and the hill? 7222 N 9584 N 35. A box resting on a 40 inclined plane experiences a normal force of 6000 N. Find the weight of the box and the force parallel to the inclined plane N 5034 N 36. A physics student on a steep hill finds himself accelerated down the hill by a force of 640 N. If the student weighs 800 N, how steep is the hill, that is, what is the angle of incline? 53 W al T Scientific